1. Field of the Invention
The present invention relates to a process and apparatus for beneficiation of minerals through froth flotation and more particularly to improvements for increasing the efficiency of column type flotation operations wherein impurities are separated from minerals and other floatable materials.
2. Description of the Prior Art
Commercially valuable minerals, for example, metal sulfides, apatitic phosphates and the like, are commonly found in nature mixed with relatively large quantities of unwanted gangue materials, and as a consequence it is usually necessary to beneficiate the ores in order to concentrate the mineral content. Mixtures of finely divided mineral particles and finely divided gangue particles can be separated and a mineral concentrate obtained therefrom by well known froth flotation techniques. Broadly speaking, froth flotation involves conditioning an aqueous slurry or pulp of the mixture of mineral and gangue particles with one or more flotation reagents which will promote flotation of either the mineral or the gangue constituents of the pulp when the pulp is aerated. The conditioned pulp is aerated by introducing into the pulp a plurality of minute air bubbles which tend to become attached either to the mineral particles or to the gangue particles of the pulp, thereby causing these particles to rise to the surface of the body of pulp and form a float fraction which overflows or is withdrawn from the flotation apparatus.
In conventional sub-aeration flotation machines the aqueous pulp ordinarily is aerated by means of a mechanical impeller-type agitator and aerator which extends into the body of pulp and which disperses minute bubbles of air throughout the body of pulp by vigorous mechanical agitation of the pulp. The feed mixture of particulate material is normally introduced into one end of a bank of flotation machines, and the agitated pulp travels or progresses in an essentially horizontal direction to the pulp discharge at the opposite end of the bank of machines. The agitated pulp, of course, becomes increasingly depleted in floatable mineral values as the pulp progresses from the feed end to the discharge end of the bank of machines. A bank of four to six mechanical cells are normally used for this purpose. Flotation machines which employ vigorous agitation of the pulp to effect aeration thereof posses serious disadvantages when employed in connection with pulps that contain difficult to float particles which, because of the vigorous agitation, may not become attached to a sufficient number of air bubbles to float the particles or which may be dislodged from the froth column lying on top of the agitated body of pulp. Moreover, when used in connection with pulps containing soft or friable particles, vigorous mechanical agitation of the pulp tends to produce slimes which in many cases adversely affect the efficiency of flotation otherwise obtainable.
To overcome these and other disadvantages of mechanically agitated flotation machines, aerating air has been introduced directly into a relatively quiescent body of aqueous pulp by means of air diffusers or aerators which are immersed in or are in direct contact with the pulp. Such flotation apparatus are commonly referred to as pneumatic flotation machines and as with mechanical cells, the flow is essentially horizontal but sometimes they have some slope. These machines have been found to be efficient when used with ores that do not require vigorous agitation in order to prevent too rapid settling out of the solid particulate matter in the aqueous pulp. They are particularly useful when the pulp being treated tends to form harmful slimes when subjected to vigorous agitation. The air diffusers or aerators of conventional pneumatic flotation machines ordinarily comprise a porous material (for example, heavy canvas, sintered metal powder structures, and the like) through which minute air bubbles are directly introduced into the aqueous pulp. As a consequence, conventional pneumatic flotation machines are subject to a very troublesome problem caused by the tendency of the air diffusers immersed in or in contact with the pulp to become covered with a tenacious coating composed of oily flotation reagents and fine particles of minerals and gangue which clogs the minute openings frustrating air flow.
Contrasted with the use of pneumatic and mechanical flotation cells are the conventional column flotation cells in which the flow is vertical instead of horizontal. Column type flotation cells and processes are described, for example, in Hollingsworth, U.S. Pat. Nos. 3,298,519 and 4,431,531, and Hollingsworth et al, U.S. Pat. Nos. 2,758,714, 3,298,519, 3,371,779 and 4,287,054.
A symposium was held on Column Flotation Jan. 25-28, 1988 at a Mining Engineers Meeting in Phoenix, Ariz. This resulted in the issuing of a book entitled "Column Flotation" in which K. V. S. Sastry was the editor. This book covers essentially all of the column development for the period 1962 through 1987.
Hollingsworth began the development of column flotation in 1952 and has continued to make developments in this field. The initial work resulted in the issuance of U.S. Pat. No. 2,758,714, dated Aug. 14, 1956. This patent describes column flotation equipment having a flared section at the top of the column which slows down the movement of the pulp and permits non-floatable material trapped in the rising pulp to drop out before it overflows the weir. The material that dropped out could be carried through a side chamber either to the bottom or midway the depth of the column. In doing so, however, some floatables would be carried along with the non-floatables and, although a grade (quality) improvement was achieved there was some reduction in recovery, but it does not overcome some faults of conventional columns. The single recycle described in U.S. Pat. No. 2,758,714 was found to result in the loss of an unacceptable amount of floatable material.
In conventional flotation columns uniform air distribution across the entire cross sectional area of the column is required to obtain good results. Should one area receive less air than other areas the downward flow in this area would greatly increase thus carrying floatable material to the bottom and out the underflow, thus reducing recovery. Uneven air distribution is probably the most serious problem encountered in conventional columns.
In prior art column flotation apparatus, efforts have been made to create a true counter-current system in which air bubbles rise straight up and pulp travels straight downward until floatable materials become attached to the rising bubbles and subsequently rise to the top of the column where they are discharged as an overflow while the non-floatables travel downward to the bottom where they are discharged as an underflow. The top overflow is usually the concentrate product and the bottom underflow is usually the waste tailings, but in some cases they can be the reverse. Recycle conditions within the column are avoided since they disrupt the uniformity of linear aeration across the cross-sectional area and tend to carry the floatable materials to the bottom where they are likely to be lost in the underflow.
"Column Flotation", a printed publication by J. A. Finch and G. S. Dobby, copyrighted 1990, discusses the undesirability of having mixing conditions within the flotation column. This publication exemplifies the past and current view which teaches away from the present invention. The authors state that "mixing has a detrimental effect upon recovery [and that] mixing also has a detrimental effect upon separation." Page 59. On page 65 the authors state that ". . . a small vertical misalignment in the column causes a large increase in axial mixing [and that] the effect of alignment has not been studied in large columns." The book does state that, ". . . circuits, particularly with recycle, have inherent advantages in terms of separation efficiency." Page 116. However, recycle in this context refers to either the drop of particles from the froth zone to the flotation zone or to the running off of a product from one column followed by a recycle through a second separation device Pp. 103-106, 132 and 134. The authors do not even suggest, but rather teach away from the concept of recycle within the flotation zone of an individual flotation column.
In most mineral beneficiation operations, it is customary to have what is known as rougher and cleaner circuits of flotation devices. In the rougher circuit a tailings product and a rougher concentrate product are produced. The rougher concentrate is then sent to one or more cleaner circuits where it is cleaned to produce a high grade final concentrate that is suitable to be marketed and a middlings product that is recycled back to the head of the circuit. In some cases the underflow tailings product is sent to a scavenger circuit to recover additional mineral values. The circuits involved can either be columns, mechanical cells or air cells and in some cases combinations of several devices.
The present invention has many advantages over conventional columns and mechanical cells, the most important of which is high recovery and high grade in a single column, thus, in many operations this completely eliminates the need for both rougher and cleaner circuits, greatly reducing both capital and operating costs.